1. Field of the Invention
The present invention relates to a radio selective calling receiver or so called a pager and, more particularly, to a radio selective calling receiver for controlling its intermittent receiving operation to realize a reduction in power consumption.
2. Description of the Prior Art
A conventional radio selective calling receiver (to be referred to as a receiver hereinafter) for receiving a POCSAG code as an asynchronous signal performs an intermittent receiving operation to prolong the service time of a battery. In the intermittent receiving operation, grouping of transmitted signals is performed, and the receiver is operated only when a transmitted signal belongs to a self-group, thereby shortening the operating time of the receiver.
FIG. 1 is a block diagram showing a general receiver. The arrangement of the receiver will be described with reference to FIG. 1.
A radio wave received via an antenna 201 is amplified by a receiving section 202 and converted into a digital signal by a demodulator 203. A control section 204 performs signal synchronization for this digital signal, reads out a self-ID from a ROM 205 in which the self-ID is stored in advance, and compares the digital signal with the self-ID.
If a self-ID is detected in the digital signal, the control section 204 sends the signal to an amplifier 206 and drives a speaker 207, thereby informing the user of the reception of the signal. A clock 208 is a reference clock used to operate the control section 204 and is also used as a sync clock. A switch 209 is used, for example, to stop an informing operation.
FIGS. 2A to 2D show the operation timings of the receiver.
FIG. 2A shows a signal to be transmitted. A preamble signal (P) used for bit synchronization is transmitted first after a state wherein no signal is transmitted (wave stoppage). A frame sync signal (SC) indicating the position of the signal is then transmitted. Subsequently, address signals classified into several groups (eight groups according to POCSAG) are transmitted. If a large number of IDs are to be transmitted, an SC signal and address signals are repeatedly transmitted.
FIG. 2B shows the timings of the intermittent receiving operation of the receiver.
In a state of wave stoppage, the receiver is operated for a short period of time to detect a preamble signal so as to detect whether a radio wave is transmitted (preamble search mode). When the receiver is activated in the presence of a preamble signal (signal repeating "1" and "0"), bit synchronization is established by this preamble signal to allow the receiver to recognize the preamble signal. The receiver is then shifted to a detection mode (SC search mode) of detecting an SC signal following the preamble signal. Upon detection of the SC signal, the position of the signal is determined. Therefore, the receiver is operated only with a self-group in which a self-ID can be set (address search mode).
A transmitted signal set in each group is subjected to error correction, and a BCH code is used to improve the reliability of the signal.
FIG. 3 shows the format of the BCH code. The BCH code is constituted by 32 bits, i.e., 21 bits as information bits, 10 bits as check bits, and one bit as an even parity bit. This signal has an inter-code distance with respect to another BHC code signal and hence can be reproduced even if an error occurs in the signal.
An address code is a BCH code. For this reason, if transmission is stopped during the address search mode, the BCH code cannot be recognized because a noise signal is received. As a result, the receiver is shifted to the preamble search mode as a mode for a state of wave stoppage.
A radio wave condition for the receiver, however, is not stable because of the influences of waves reflected by buildings and the like. For this reason, in spite of the fact that a signal is actually transmitted, a BCH code may not be recognized because of a weak electric field, and the receiver may be shifted to the preamble search mode. In this case, in spite of the fact that the address follows the SC signal, the receiver can hardly search for a preamble signal. Consequently, the receiver cannot detect the preamble signal, and hence cannot detect the signal belongs to the self-group. Even if, therefore, a self-ID is set in the self-group in this state, the receiver cannot receive the signal. For this reason, a middle synchronizing function is provided for the receiver to quickly restore to the address search mode.
FIGS. 4A to 4C show a synchronization method. Bit synchronization and middle synchronization will be described below with reference to FIGS. 4A to 4C.
FIG. 4A shows a received signal. As this received signal, a preamble signal (signal repeating "1" and "0"). FIG. 4B shows a clock for bit synchronization performed by the control section 204. FIG. 4C shows window frames arranged in the control section 204 to establish middle synchronization.
Bit synchronization will be described first. Consider the received signal shown in FIG. 4A and the bit synchronization clock (twice as fast as the received signal). The logic of the bit synchronization clock at the time when the logic of the received signal changes is determined. The bit synchronization clock is controlled as follows. If the bit synchronization clock is at "L" at this time, the duration of the clock is reduced by a predetermined time. If the bit synchronization clock is at "H", the clock is advanced by a predetermined time. In this case, since the bit synchronization clock is kept at "L"(until X.apprxeq.0), a value X is gradually reduced. Bit synchronization is established at a time point at which the value X approaches "0" (falls within a certain range).
Note that the received signal is determined at the trailing edge of the bit synchronization clock. That is, when bit synchronization is established, the leading edge of the received signal coincides with that of the bit synchronization clock. Therefore, the trailing edge of the bit synchronization clock coincides with a middle point of the received signal, allowing accurate reading of reception data.
Middle synchronization will be described next. Window frames, each having a predetermined width equivalent to 1/2 the baud rate of the receiver, are prepared in the receiver. The first window frame is arranged at the leading edge of the received signal. It is then checked how many leading edges of the received signal appear in the subsequent window frames. When a predetermined number of leading edges is checked, middle synchronization is established, and the SC search mode is set (if the received signal is a signal of the baud rate of the receiver, all the leading edges appear in the window frames). That is, middle synchronization can be established during reception of an address signal or the like.
As described above, if only the conventional middle synchronization method is used, a change point of noise may appear in a window frame, resulting in erroneous middle synchronization. In this case, since a battery saving operation as shown in FIG. 2C is performed, the SC search mode time is prolonged. That is, the power consumption increases, thus posing a serious problem.